1. Field of the Invention
This invention relates to oil and gas production well stimulation where said wells are low-producing or have ceased to produce by primary and secondary recovery means.
2. Description of the Prior Art
Certain phosphate esters have been used in conjunction with water-flood methods of driving oil toward producing wells as a means of secondary recovery of hydrocarbons. The phosphate esters are added to an injection well and perform various functions such as scale inhibition, and the formation of a slug for driving the oil in the formation toward the producing well. The phosphate ester can be injected into the well either as an aqueous solution or as a soluble oil micro-emulsion. Alkyl and aralkyl polyoxyalkylene phosphates are disclosed as useful surfactants in water-flood secondary recovery processes. Such processes are disclosed in U.S. Pat. Nos. 3,435,898, 3,596,715, and 3,480,083.
Producing oil and gas wells have long been treated to stimulate production thereof utilizing a method termed "acidizing" in which an emulsion of an aqueous mineral acid either alone or in combination with various surfactants, corrosion inhibiting agents, and hydrocarbon oils is added to a producer well. Presumably, such treatments tend to remove deposits from the area of the subterranean oil or gas formation immediately adjacent to the production well bore, thus increasing the permeability of the formation and allowing residual oil or gas to be recovered through the well bore. Another object of such "acidizing" treatment of oil or gas producer wells is the removal of water from the interstices of the formation by the use of a composition which materially lowers the interfacial forces between the water and the oil or gas. Various surface-active agents have been recommended for this use.
Usually, the surface-active agent is injected into the production well in combination with crude oil or other hydrocarbon solvent followed by the injection of additional crude oil to move the treating fluid into the formation. U.S. Pat. Nos. 3,467,194 and 3,470,958 provide specifically for the treatment of a crude oil production well utilizing an oil-external micellar dispersion or a water-external micellar dispersion followed by the injection of a hydrocarbon to move the micellar dispersion outwardly from the well bore into the formation. There is disclosed in U.S. Pat. No. 3,620,303 a method of treating a production well with an aqueous solution of an anionic orthophosphate ester surfactant. The aqueous surfactant solution is retained in the formation at least about 24 hours and thereafter the solubilized hydrocarbon is displaced toward the producer well by fluid drive means utilizing a second well bore which is in fluid contact with the underground oil formation. It is theorized that the surfactant solution forms a soluble oil micro-emulsion with the formation hydrocarbons.
In U.S. Pat. No. 4,278,129, there is disclosed a method of increasing production or returning to production oil and gas wells by the treatment of a production well comprising adding an aqueous solution of an oxyalkylated phosphate ester surfactant and thereafter adding a hydrocarbon to drive the aqueous surfactant solution into the formation a distance of about 3 to about 20 feet from the well bore. The aqueous surfactant solution is allowed to remain in the formation for an effective period generally not less than 24 hours and preferably 24 hours to 1 month. Thereafter, the oil or gas is recovered using pump and/or conventional fluid drive means wherein water is injected into an injection well in communication with said oil or gas subterranean formation.
There is no indication in any of the prior art references that solutions and dispersions of oxyalkylated phosphate ester surfactants in non-aqueous solvents would be useful in returning to production oil and gas wells which are low-producing or have ceased to produce. The method of the invention is particularly useful in the stimulation of oil and gas wells which have failed to respond to acidizing treatment of the producing well including the use of various acids with various surfactants, and may also be used effectively in the treatment of water-sensitive formations.